A speed of transfer of data of 4 M to 8 MB/second is sufficient for handling data base processing and desk top publishing (DTP), but when handling image data having a large amount of data, a further higher speed of transfer of data is necessary. Namely, when fetching full color image data consisting of 640 pixels.times.480 pixels, a speed of transfer of data of about 26 MB per second is required, but hard disk drives of current personal computers cannot cope handle such a speed of transfer of data. For this reason, in hard disk drives, to enable handling of data of a high transfer speed such as image data, a disk array system was developed.
This disk array system is constituted by for example a first hard disk drive (HDD1) to a fifth hard disk drive (HDD5) as a plurality of hard disk drives. The recording of the data is carried out by RAID (Redundant Array of Inexpensive Disk) 1 to RAID5 recording systems defined for individual functions and individual performances. Note that, among the RAID1 to RAID5, the RAID1, RAID3, and RAID 5 systems have actually been used as recording systems.
More specifically, when recording one series of data consisting of data 1, data 2, data 3, . . . attaching importance to safety, the recording of the data is carried out by using a mirroring system, that is, the RAID1 recording system. This mirroring system is designed to record the same data of the data 1, data 2, data 3, . . . by each of the first to fifth hard disk drives. Due to this, even in a case where trouble occurs in for example the first hard disk drive and the series of data was not correctly recorded, the same data as that for the first hard disk drive is recorded by each of the remaining second to fifth hard disk drives and therefore the safety of the data can be secured.
Next, when recording the series of data attaching importance to the speed of transfer of data, the recording of the data is carried out by using a striping system, that is, one of the RAID3 or RAID5 recording systems.
The RAID3 striping system is designed to use a specific hard disk drive among a plurality of hard disk drives as a hard disk drive for parity data. In this case, the fifth hard disk drive is used as the hard disk drive for the parity data. This RAID3 striping system is designed to record the data 1, data 5, data 9, . . . by the first hard disk drive; record the data 2, data 6, data 10, . . . by the second hard disk drive; record the data 3, data 7, data 11, . . . by the third hard disk drive; and record the data 4, data 8, data 12, . . . by the fourth hard disk drive. The parity data of the data 1 to data 4, the parity data of the data 5 to data 8, the parity data of the data 9 to data 12, . . . are recorded by the fifth hard disk drive. Due to this, the series of data can be recorded in parallel by the hard disk drives, and therefore the system can handle data of a high transfer speed.
Next, the RAID5 striping system is designed to record the parity data dispersed among a plurality of hard disk drives. This striping system of RAID5 is designed to record the data 1, data 2, data 3, and data 4 by the first hard disk drive to fourth hard disk drive and, at the same time, record the parity data of the data 1 to data 4 by the fifth hard disk drive; record the data 5 to data 7 and the data 8 by the first hard disk drive to third hard disk drive and by the fifth hard disk drive, respectively, and, at the same time, record the parity data of the data 5 to data 8 by the fourth hard disk drive; record the data 9 to data 10 and the data 11 and data 12 by the first and second hard disk drives and by the fourth and fifth hard disk drives, respectively, and, at the same time, record the parity data of the data 9 to data 12 by the third hard disk drive. Due to this, the series of data can be recorded in parallel by the hard disk drives and therefore the system can handle data of a high transfer speed.
In the RAID3 recording system, the parity data is recorded only by the fifth hard disk drive, and therefore the writing of the data cannot be carried out simultaneously by different hard disk drives, but in the RAID5 recording system, the parity data is recorded dispersed among the hard disk drives and therefore the writing of the data can be simultaneously carried out by the different hard disk drives and the overall performance can be improved.
The RAID1, RAID3, and RAID5 recording systems used in such a disk array system can be selected in accordance with the data to be recorded or the desire of the user. For this reason, image data which requires a high speed of transfer of data may be recorded by the RAID3 or RAID5 system which can handle a high speed of transfer of data.
However, in such a conventional disk array system, while since the first to fifth hard disk drives of the plurality of hard disk drives can be used connected with each other and therefore the virtual capacity is infinitely large, in practice, a single hard disk drive system is constituted by the first to fifth hard disk drives and therefore the capacity thereof is restricted to that of five hard disks. Also, for example even if 100 of these disk array systems are connected and used, each system is independent, and therefore it becomes difficult to comprehensively control them and therefore an increase of capacity still cannot be achieved. Further, even in a case where trouble occurs in just one hard disk drive, it is necessary to exchange all of the hard disk system and therefore it cannot be said to be a removable (easily partially exchangeable) system.
On the other hand, known in the past has been a "juke" device having a single magneto-optic disk drive and a disk changer unit for loading or removing a plurality of magneto-optic disks to and from the magneto-optic disk drive. This juke device writes data on the magneto-optic disk loaded in the magneto-optic disk drive. When finishing writing the entire capacity of the magneto-optic disk, the disk changer unit exchanges the magneto-optic disk loaded in the disk drive with a new magneto-optic disk and continues the recording of the data. For this reason, this juke device enables the overall storage capacity to be made infinitely large by recording data taking out the magneto-optic disks which have finished being written on and successively loading new magneto-optic disks in the disk drive.
Here, since the juke device records the data by a single disk drive, there is a concern that the speed of transfer of data would be restricted to the data processing speed of the disk drive and that therefore data of a high speed of transfer could not be handled. For this reason, the juke device is provided with a hard disk which temporarily stores the supplied data, uses this as a buffer memory, and records the data read from the hard disk on the magneto-optic disk, thereby handling data of a high transfer speed.
However, since this juke device records data by a single disk drive, if a trouble occurs in the disk drive, it becomes impossible to record the data. On top of this, replacement entails replacement of the entire disk so this also could not be said to be a removable system. Also, even if a hard disk is used as a buffer memory, current hard disk drives cannot handle data of further higher transfer speeds such as the image data as mentioned above, and therefore there are limits to the transfer speeds of the data which can be handled.